Update note about confirmation emails in publishing workflow docs

Why these changes are being introduced:

The readme indicates that the last email to look for while running
the publication workflow is the preservation email. The past few
times I've run the job, it's been the MARC export email.

Relevant ticket(s):

N/A

How this addresses that need:

This generalizes the aforementioned note to check for the presence
of all three emails, before suggesting that the MARC email may
be the last one in the series.

Side effects of this change:

None.

README.md

@@ -375,8 +375,9 @@ Publication Review - Publish)
   # run the output queue processing job
   heroku run -s performance-m rails dss:process_output_queue --app TARGET-HEROKU-APP
 
-  # wait for all ETD emails to be received (the preservation email is the final one to look for)
-  # scale the worker back down so we do not pay for more CPU/memory than we need
+  # wait for all ETD emails to be received (there are three emails: one overall results summary, one preservation
+  # results summary, and one MARC batch export).
+  # Then, scale the worker back down so we do not pay for more CPU/memory than we need
   heroku ps:scale worker=1:standard-1x --app TARGET-HEROKU-APP
   ```